Objective: Our overall aim is to determine the radiation dose-rate dependence of tumor cure in the radiotherapy of mouse tumors in comparison with the dose-rate dependence for causing damage in adjacent normal tissues. Values of the TCD50 (tumor cure dose for 50 percent of animals) will be determined for several dose rates, radiation qualities, fractionation schemes and tumor types. The resulting dose-rate dependence of TCD50 under these conditions will be compared with dose- rate relationships for radiation injury to normal tissues. Approach: Selected types of transplanted tumors in mice are irradiated locally in various dose, dose-rate, temperature and fractionation groups using orthovoltage or supervoltage radiation. Observations of tumor cure, skin telangiectasia, skin necroses and fibrotic decrease in ankle flexion are analyzed by probit analysis to determine TCD50's and ED50's (50 percent effect doses which caused half of the animals to show each of the normal tissue and end points). The therapeutic ratio (TR) can then be calculated as the ratio of ED50 to TCD50 for each dose rate.